Surface metal welding process and apparatus

ABSTRACT

Material is melted in the surface of a metal body by maintaining a plurality of 300 to 10,000 ampere electric arcs, each energizing between an electrode surrounded by a gas cup or nozzle and such surface. The nozzles are positioned about 1/4 to about 1 gas cup diameter from the surface. The arcs are swept across the work by self-induced electro-magnetic interaction while providing relative motion between the surface and the arcs. The speed of such motion is correlated with the current of such arcs.

I United States Patent [15] 3,673,374 Hauck 14 1 June 27, 1972 [54]SURFACE METAL WELDING PROCESS 2,806,124 9/1957 Gage ..219/121 ANDAPPARATUS 3,050,616 s/1962 Gage ..219 121 2,927,990 3/1960 Johnson.......219/76 X [72] Invent muck some, NY 2,330,289 9/1943 Keir 1 ..2l9/76[73] Assignee: Union Carbide Corporation, New York 2,003,019 5/1935Strobel ..2l9/76 Primary Examiner.l. V. Truhe [22] Flled' 1970 AssistantExaminer-George A. Montanye [21] App1.No.: 30,947 AttorneyPaul A. Rose,Harrie M. Humphreys and Dominic .l. Terminello Related US. ApplicationData [63] Continuation-impart of Ser. No. 701,932, Jan. 31, [57]ABSTRACT 1968 abandoned whfch a continuanon'm'pm of Material is meltedin the surface of a metal body by maintain- Apnl 1964 abandoned ing aplurality of 300 to 10,000 ampere electric arcs, each energizing betweenan electrode surrounded by a gas cup or [2?] ..219/7i3,223lk913'l nonleand such Surface The nozzles are positioned about 14 i 121 to about 1gas cup diameter from the surface. The arcs are 1 w o are swept acrossthe work by self-induced electro-magnetic in- 56] References Citedteraction while providing relative motion between the surface UNITEDSTATES PATENTS 8/1967 Hauck eta] ..219/l21 and the arcs. The speed ofsuch motion is correlated with the current of such arcs.

8 Claim, 7 Drawing Figures P'A'TE'NTEnJum 19-12 3.673 .374

' sum 10F 3 Carbon Steel Slab l fi TI Coated with y l leto GranularFerrochrome Process Second Side l (Optional) Preheated to I lsoo'r I l ll l Arc Melted I 1 to Alloy and Bond 1 l IE EL l Shot Blast Inspect CropEnds Hot Roll Descale Plate Product AT/TORNEY PATENTEDJUH 27 I972 3. 673 1374 saw 3 BF 3 INVENTOR FIG 7 gj gmaonwucx & p

ATTORNEY SURFACE METAL WELDING PROCESS AND APPARATUS This application isa continuation-in-part of a application Ser. No. 701,932 filed Jan. 3 l1968, now abandoned, which is in turn a continuation-in-part ofapplication Ser. No. 359,952, filed Apr. 15, 1964, and now abandoned.

This invention relates to a process for are melting material in thesurface of metal bodies and more particularly to a process for claddinga metal body with a surface layer of a dissimilar metal.

As one example, the invention provides for substantially eliminatingsurface defects in a body of metal in the solid state, which comprisesenergizing each of a plurality of electric arcs with high current, i.e.,a value of at least of the order of 300 amperes, flowing between agas-shielded electrode and such surface. The adjacent surface metalincluding such defects is melted with such arcs each in an annularstream of gas which further protects the arcs and the so-melted metalfrom contamination by the atmosphere, while sweeping such arcs acrossthe work by self-induced electro-magnetic interaction. This sweepingaction not only stirs the melt without adversely blowing it away fromthe solid metal thereunder, but actually retains the molten metal in asubstantially smooth and level state to effect a wide coverage of themetal surface. A protective gas blanket is also provided at least untilthe metal becomes completely solidified, before it is exposed to theatmosphere.

High-chromium alloys require careful. surface conditioning prior tohot-rolling in order to insure satisfactory surface quality. Because oftheir inherent resistance to oxidation, such alloys do not scalereadily, hence they retain surface defects which normally are eliminatedin the reheating furnaces in the case of more freely scaling metals.Most of the ingot defects causing surface rejections are laps and seamsdue to metal splashing. Since they are located near the surface, thestandard commercial practice is to remove these defects mechanically bygrinding with abrasive wheels. More recently, powder scarfing techniqueshave been developed. However, both processes result in considerable lossof metal.

As another example, the invention provides for melting surface material,such as a preselected composition of powdered material secured by asuitable adhesive to the surface of a metal slab, whereby such materialis alloyed with and fused into the base metal. Chromium, for example,could be so alloyed with a carbon steel slab to form a stainless steelalloy at the outer surfaces.

Various processes have been proposed for providing a product having askin or clad coating of a corrosion resistant quality surface appearancematerial on a core of a low cost structural material. An example of aparticularly useful composite product of this type would be sheet, plateor other stock having a carbon steel core or body and a tight, adherentcladding of stainless steel. This material would presumably have thehigh surface appearance and corrosion resistance of stainless steelcombined with the strength, ductility and low cost of the carbon steelcore. Of equal interest would be a practical process for making suchcomposite material.

The high cost of stainless steel sheet is due in large part to theexpenses of its difficult fabrication. Attempts have been made toproduce a stainless clad-carbon steel slab which could then be rolled tosheet form using conventional carbon steel rolling practice and in thatway provide a cheaper composite stainless clad-carbon steel sheetproduct having many of the properties of a fully stainless sheet.

A more desirable process, if it could be effectively practiced, would beto provide a stainless steel coating on a carbon steel slab by meltingstainless alloy constituents, e.g. chromium and nickel, etc. into thesurface of a carbon steel slab. Assuming that a uniform depth of thestainless clad layer could be produced on the steel slab, this uniformand integral slab could then be expected to undergo fabrication intosheet and other stock with more success than laminated bodies havingseparate sheets and slabs of different materials. Another advantage ofsuch a process is the use of alloy metal (e.g., Cr.) in

low cost form rather than as expensive semifinished stainless plate.However, the provision of such a starting material, i.e., a carbon steelslab clad with an integral or melted-in alloy coating or layer, is anextremely difficult proposition. Electric arcs have been cited as beingsuitable heat sources for such a job, but a practical process has notbeen heretofore provided.

It is a main object of this invention, therefore, to provide an improvedprocess for are melting material in the surface of a metal body.

Another object is to provide an improved process for producing an alloysurface layer on a metal body.

It is a further object of this invention to provide a process forproducing an alloy surface layer on a metal body, which composite bodywill have the properties and structure allowing low cost processing intosheet, plate and strip of exceptional quality.

It is also the object of this invention to provide a process forproducing arc-alloyed slabs by means of a highly efficient arc systemand alloying process.

It is also an object of this invention to provide an arc-alloyed slabhaving one or more surface layers formed thereon of a materialdissimilar to the base, and having the structure, uniformity and otherproperties foundnecessary to allow its processing into high quality cladsheet, plate and strip.

It is a further object of this invention to provide an improvedapparatus for producing an alloy surface layer on a metal body using anarc melting system.

Other aims and advantages of this invention will be apparent from thefollowing description, the drawings and the appended claims.

In accordance with these objects a method is provided for alloying amaterial into at least a selected portion of the surface of a body of abase material wherein a part of the base material becomes a part of thealloy and wherein an alloy layer of substantially uniform depth andcomposition is formed on said selected portion of the surface of saidbody, comprising melting a selected continuous lateral expanse of thesurface of said body onto which alloying material is introduced byestablishing a plurality of arcs at current levels of from 300 to 10,000amperes from electrodes provided in are devices which include a nonle orgas cup having an outlet passage, confining the arc in the nozzle,introducing gas through the nozzle and into the arc, the combination ofsaid gas flow and said outlet passage nozzle diameter providing adirectionally stable arc effluent; positioning said directionally stablearc effiuents proximately to each other so that the magnetic fieldscreated around each of said arcs efiects the arcs in immediate proximitytherewith, energizing said arcs such that an AC voltage is applied atleast to alternating arcs to produce said interacting magnetic fieldsamong said multi-arcs, spacing said nozzles from about /4 to about 1nozzle diameter from the surfaces of said body such that the length ofsaid arcs may be swept across said selected surface portion and varyingat least one of said are current, said gas flow, and said arc length tocontrol the sweep of said arcs, and moving said body and sail pluralityof electric arcs relative to one another in a direction from one end ofthe body to another to advance the molten zone of alloyed material tosuccessive lateral expanses on said body, while allowing already moltenlateral expanses to cool under conditions substantially uniform acrossthe entire selected lateral expanse.

in a broader aspect the objects are accomplished by a method of meltingmaterial in the surface of a metal body by maintaining a plurality of300 to 10,000 amperes electric arcs each energized between an electrodesurrounded by a gas cup or nozzle and such surface, positioning suchnozzles about V4 to about 1 nozzle diameter from said surface, meltingonly the adjacent surface material with such arcs each in an annularstream of gas which further protects the arc and so-melted material fromcontamination by the atmosphere, and sweeping such arcs across the workby self-induced electro-magnetic interaction to retain, stir and levelthe so-melted material, while providing relative motion between saidsurface and the arcs the speed of such motion being correlated with thecurrent of said arcs.

In the drawings:

FIG. 1 is a chart outlining typical process steps involved in producingarc-alloyed slab (enclosed in the box), followed by a rolling schedulefor producing clad plate or sheets according to this invention;

FIG. 2 is a somewhat schematic diagram, in cross section, of an arcalloying operation according to one aspect of this invention;

FIG. 3 is a fragmentary plan view of one form of apparatus suitable forcarrying out the process of this invention;

FIG. 4 is a schematic representation of an array of gaseous arcproducing torches as employed in the preferred embodiment of thisinvention;

FIG. 5 is a fragmentary view in partly transverse cross section takenalong line 55 of FIG. 3;

FIG. 6 is a sketch of a cross section of an as-alloyed" body showing aclad layer produced according to the process of this invention;

FIG. 7 is a sketch of a cross section of an as-alloyed" body showing apossible defective structure;

Referring to the drawings, FIG. 1 shows a chart of the various processsteps typically involved in making clad plate or sheet product. Thesteps relating to the 'arc alloying process for forming the cladding onthe slab are enclosed in the box. The subsequent steps relate to typicalprocedures for converting such slab to wrought product, namely hotrolling, cold rolling, etc.

According to the process, a carbon steel slab flat enough to hold themolten metal puddle which is to be formed thereon is first provided.Where the clad product will be sheet, a mild steel such as a rimmed orkilled steel is generally used because of its good formability andmechanical properties. Higher carbon, firebox grade steels may beemployed where clad plate is desired. Slight thicknesses of scale on theslab or minor irregularities in contour will not generally interferewith the are alloying process and may be tolerated.

The alloy constituent materials, generally in a granular form are thendeposited on a surface of the slab. Where the objective is to form aferritic stainless clad on the slab, then a chromium containing materialis used, e.g. a ferrochromium alloy. Of course, other alloy compositionscan be formed where needed or desired. For example, a precalculatedmixture of ferrochromium and nickel can be used to make an austeniticclad. To form the ferritic, type 430 stainless clad, for example, aSimplex No. 2 ferrochrome of a particle size generally 8 mesh and downmay be used. This material has the typical composition shown below.

TABLE I As seen above this type of ferrochrome has a chromium content ofabout 70 per cent whereas if a straight chromium A stainless is desired,the clad layer should have a chromium Final ercent Gr desiredxclad depth[111.]

p Xclad density [lb.lin.

Fer ce nt Cr in ferrochrome used Amount of ierrochrome to be added inlbs./li:\. of slab surface The depth of the clad layer desired will, ofcourse, depend on such factors as the type of base material and cladcomposition involved, slab size and the intended use of the slab, i.e.,is it to be processed into sheet, or thick plates, etc. Generally,however, in regard to stainless cladding of carbon steel slabs forrolling into sheet stock, the depth of the clad layer should be about 10per cent of the slab thickness. For a 5 inch thick slab then, the depthof cladding to be produced would be about 0.5 inch. If an 18 per centchromium clad is to be provided, then a layer of ferrochrome providingabout 0.036 lb/in would have to be deposited over the slab surface.

To aid in holding the granular ferrochrome on the slab a suitable binderis employed. For example, a dilute solution of sodium silicate can besprayed on the slab surface before the ferrochrome is applied, followedby another spraying of sodium silicate binder after the deposition ofthe granular material. It is to be noted that the alloy constituentmaterial may be provided on the slab surface in forms other thangranules. A sheet of a chromium containing material may be placed overthe slab surface, or wires, bars, rods, etc. may be used. The granularmaterial, however, appears to be best suited to the process.

The ferrochrome-coated slab is usually preheated to a suitablepredetermined temperature. Some preheating of the slab is generallyrequired to prevent cracking or separation of the clad layer. The degreeof preheating desired for a particular slab material ,and cladcomposition will vary depending on several factors. Preheating to a hightemperature in a fuelfired preheating furnace is generally desired sincethis will reduce the amount of electric energy needed to melt and alloythe clad layer and will also promote uniformity. For the stainless cladcarbon steel being considered here, a preheat temperature of l,500 F.was satisfactory, although it can be significantly higher or lower.

While the description of the process outlined above has shown theapplication of the alloy material to the slab before preheating, thealloy material could also be applied after preheating or even during theactual melting process by using suitable material deposition equipment.

The alloy material could be, for example, chromium, nickel, iron,columbium, manganese, molybdenum, tantalum and titanium.

The slab is then delivered to the arc melting station where the arcarray shown in the drawings and described hereafter is used to melt thealloy constituent material and underlying portion of the slab itself toform the desired depth of clad layer. While arc melting processes havebeen contemplated in the past, it is the provision of the improvedmelting and alloying process used here that makes it commerciallypossible to obtain composite slabs with clad layers of uniform depth andalloy content as well as the internal structure necessary for goodfabrication.

While electric arcs have been used in the past, both singly and ingroups, for welding, metal surface treatings, cutting and melting,including surface melting, the electric arc melting and alloying systemused herein is particularly adapted to producing a clad slab of the typefound necessary for subsequent processing into plate or sheet stock. Ithas been found that a continuous puddle of molten material formed acrossthe entire lateral expanse of the surface to be treated is preferredrather than allowing the formation of a number of overlapping butseparate narrow melt puddles. The system of this invention isparticularly adapted to the production of said continuous well mixedmelt puddle because of its uniform intense heating of the metal surfaceand thorough stirring of the alloying and base metal.

While it might seem that uniform heating of a large metal surface couldbe achieved by merely placing a large number of electric arc producingdevices together, such an operation is not easily conducted. Electricarcs, including ordinary gasshielded electric arcs, tend to be blown,deflected or otherwise undesirably influenced by random magnetic fieldsand drafts. These unstable arcs are thus uncontrollable andunpredictable, especially when grouped together in close proximity. Ac-

cording to the process of this invention, a plurality of in-,

dividual directionally stable electric arc columns are arranged toextend over the lateral expanse to be treated. A plurality of thesedirectionally stable arcs can be arrayed over the surface of the slaband positioned so as to obtain an electromagnetically controlledsweeping of the arcs across any selected lateral surface of the slab,producing, in effect, a substantially continuous, sheet-like plasmaflame which will uniformly heat, melt and stir said selected expanse ofthe slab at one time, thereby producing a clad layer on said slab of thedesired uniform depth, composition, and solidification structure. Thesheet-like plasma flame is then moved relative to the slab in adirection from one end of the slab to the other to advance the moltenzone of alloyed metal to successive lateral expanses on said body, withthe result that already molten zones of lateral expanse cool undersubstantially uniform conditions creating a clad layer of optimummetallurgical conditions for subsequent rolling and other metal formingoperations.

Generally speaking, whether it be for surface conditioning or foralloying material into the surface of the body the arc torches should beplaced as close to the work as possible for more efficient heattransfer, and for more effective utilization of the arc gas. However,because of the sweeping nature of the arcs, the amount of stand-off iscritical. If the gas cups or nozzles of the arc torches are too close tothe work, the arcs will not sweep. On the other hand, if the standoff istoo great, the arcs will attach to the adjacent gas cup. Expressed as afunction of the gas cup diameter, it has been found that the standoffdistance should be between V4 and 1 gas cup outside diameter.

The phrase directionally stable" when used herein and in the claimsdescribes an electric arc column in which the longitudinal axiscoincident with the flow of current remains substantially invariant indirection regardless of the surrounding environment such as air drafts,and regardless of intentional relative movement of the arc columns andthe workpiece, expect that in the presence of intentionally providedmagnetic fields, such are columns can be deflected or swept as desiredto form the sheet-like plasma flame.

There are several methods of producing and maintaining the arcs usefulin the process of this invention. One such method is disclosed andclaimed in U.S. Pat. No. 2,806,124 issued Sept. 10, 1957 to RM. Gage. Asseen in the schematic of F IG. 2, this method comprises establishing anare from a non-consumable electrode 12, introducing a flow of gas suchthat at least a portion of the gas stream is directed by means of a coldwall nozzle 16 into intimate contact with the are thereby directionallystabilizing the arc 10. The slab l8, coated with a granular ferroalloy20 and generally in a preheated condition, is moved under the are 10.The heat of the arc melts the granular ferroalloy as well as a portionof the underlying slab metal. The molten metal from the slab is alloyedwith the ferroalloy material forming the clad alloy layer 22 which coolsand solidifies as it leaves the arc melting zone.

It has been found that a directionally stable arc, such as that producedby the process described in the afore-mentioned patent, is essential forthe successful operation of the process here. The directionally stablearc will not wander over the surface of the slab, as will other types ofarcs, but will have stiffness and persistence of direction.Additionally, the directionally stable arc column produced as describedin the above mentioned patent can be placed closer to an adjacentdirectionally stable arc column, e.g. as close as about 1% inches, toincrease the melting power without danger of arcing to the adjacentdevice. As will be explained more fully hereafter, open arcs cannot beplaced sufficiently close to provide the melting capacity required forthis process because of the double arcing tendency of closely situatedopen arcs, and the tendency for uneven melting due to undesirable arcposition resulting from magnetic interaction.

The arc torch device described in the aforementioned patent has thefurther advantage that the arc-gas effluent issuing therefrom is of highenergy density compared to that of an equivalent open are or ordinarygas shielded arc. This arc-gas efiluent thus is capable of faster andmore uniform heating and melting of the alloy layer because of itsincreased energy intensity.

Additionally, the gas flow in said arc-gas effluent serves a usefulpurpose in protecting the molten puddle and in stirring the alloymaterials into the molten portion of the slab as well as pushing awaymolten material to expose fresh, unmelted areas of the slab for thearcs. However, the amount of gas flow is not so much as to cause severesplashing of the molten metal.

When an arc melting operation was attempted using a row of devicesproducing open arcs, i.e. non-constricted arcs, it was found that theoperation of the system was particularly unstable. The are columns werestrongly attracted or deflected for are column spacings of 1% inches to2% inches. Increasing the device spacing and decreasing standoffdistance over the workpiece was necessary to minimize such arcinteractions which, if left uncontrolled, would cause arcing from onedevice directly to an adjacent device with resulting destruction of theare devices. It was found with the use of open are devices in the rangeof 200 to 400 amperes are current, that spacings greater than 2% inchesare required to avoid undesirable arc deflections and device failures.The need for excessive spacings makes it impossible to achieve uniformmelting by intense heating and stirring of the conditions across lateralexpanse of the slab to be treated. For example, in one prior art systemfor melting a slab using three open arcs at about 800 amperes each, aspacing of 7% inches was used. This arrangement generates about 5 kw ofenergy per inch of pass width and can only proceed at a forward speed ofabout two-thirds inch per minute. Such a slow rate of forward progresscauses wide temperature difierentials in the body being melted, i.e.,the portions already melted cool and solidify while only slightlyforward spaced areas of the body are being melted. Such conditions arenot conducive to formation of a clad body having a uniform alloyconcentration and solidification structure. Additionally, when largelateral spacings are utilized, as required with open arcs, the areasbetween the arc columns are not all heated at the same time, even whenthe open arcs are mechanically oscillated in a sideto-side motion. Ithasfurther been found that merely increasing the number of rows ofwidely-spaced open are devices would not contribute useful energy inproportion to the increase in total are power. This is due to the needfor maintaining relatively large spacings between the rows of open aredevices. Only a slight increase in forward speed would result and therewould be a tendency for the melt between the rows to freeze so that theonly contribution made by the preceding row would be that of preheating.

The use of directionally stable, high intensity electromagneticallyswept arcs of the type herein makes possible the practical attainment ofthe high capacity melting system needed to uniformly melt the entirelateral expanse of the slab and thereby produce a composite body fullyamenable to further fabrication.

However, it is to be noted that the invention is not to be consideredlimited to use with the particular directionally stabilized arc efiluentproduced according to the process described in the above-cited patent.The use of any device or system which will produce a directionallystable and/or constricted arc-gas effluent is within the scope of thisinvention.

An example of an arc alloying apparatus used in the practice of thisprocess is shown in FIGS. 3, 4 and 5. This apparatus was used to arealloy portions of slabs having a typical size as follows: 18 inches wideX 34 inches long and 5 inches thick. The are alloying of largercommercial slabs requires equipment scaled up in size but of the samegeneral type and arrangement.

As seen in the drawings a shielding box 28 about 10 inches long and 16inches wide encloses a group of 18 torches T arranged in three rows orarrays of six torches each. Referring back to FIG. 2, the torches shownconsist of a water-cooled l percent-thoriated tungsten electrode 12 ofabout one-fourth inch diameter surrounded by a constrictingwater-cooled, copper gas cup or nozzle 16. The outside diameter of thegas cup is about seven-eights inch diameter and the orifice diameter isabout five-sixteenths inch diameter. The electrode is set back aboutone-fourth inch from the nozzle orifice. These torches have a rating ofabout 400 amperes and when operated at 330 amperes have a powerrequirement of 12.5

kw each or 225 kw for the 18 torchesl The torches may be spaced about 1%inches center to center from each other. A forward speed in excess of 3inches per minute can be obtained using a torch arrangement of thistype. A rapidly moving system of this type allows for more uniformmelting and solidification of the treated body.

The three rows of torches are shown immediately behind one another butthe rows could be staggered to give more uniformity. The slab 18 to beprocessed is moved under the box along a horizontal path on suitablerolls 30 or other suitable means. A suitable gas source such as argon isconnected at 32 to the box 28 for passage therethrough around thetorches and over the slab. This assures shielding of the molten metalunder the arcs. Additionally, a housing 34 may follow the torch box 28so as to continue the protective gas shielding of the cooling metalafter it has been melted and alloyed. This housing 34 may have ashielding gas inlet 36 and a diffuser 38, see FIG. 5, such as ahorizontal partition of porous material dividing the interior into anupper gas inlet chamber 40 and a lower gas outlet chamber 42. Anyarrangement of such protective gas shields are suitable provided theygive a uniform distribution of the gas in the form of a protectiveblanket over the molten metal as it cools and solidifies. Alternatively,a granular flux may be employed to shield the molten puddle, as forexample the case of submerged arc welding.

The slab is shown enclosed at each side by graphite block dams 50. Thesedams serve to prevent molten metal from running off the slab, especiallywhen the cladding is to extend across the entire lateral expanse of theslab. The dams also improve the gas shielding of the molten metal bydirecting the gas flow uniformly out along the edges of the box.

In some cases, it may be desired to are alloy only a section of a slab,for example, a section not extending across the full width of the slab.The untreated edges can be sheared off later leaving only the treated orclad section. In such cases the term entire lateral expanse as usedherein means only that portion of the width of the slab which isintended to be treated. The step of providing a continuous melt puddleacross the entire lateral expanse of the slab then means forming thecontinuous melt puddle across the actual width intended to be treated.

As shown in the FIG. 3, the torch box 28 can be moved in a side to sidedirection as the slab travels under the arcs. The oscillation of thetorches will further insure uniformity of cladding and serve to increasethe pass width allowable with a given arrangement of torches.

Oscillation can be obtained using a rotating drive wheel located in anoscillator 44 with an off center hole and push rod 46 which moves thetorch box assembly back and forth on a track transverse to the slabtravel direction. In regard to the 3 X 6 torch array shown, anoscillation amplitude of 2% inches and a frequency of 19 cpm was foundeffective.

The directionally stable torches can be operated using alternatingcurrent or direct current, straight or reverse polarity. In theschematic diagram of FIG. 2, the single torch T is shown operating ondirect current. An electrical connection to the workpiece is required.

The directionally stable arcs can also be operated with alternatingcurrent. FIG. 4 shows a particularly useful arrangement for ac.operation of the torches whereby a controlled sweep of the electric arcsis obtained and whereby no electrical connection to the workpiece isrequired. As seen there, A, B and C represent vectorially sources ofthree voltages each 120 out of phase with respect to the others; i.e., aconventional threephase power source. If the leads to the coils of asimilar threephase source, A, B and C are transposed as shown, asixphase power source is created, with each voltage source 60 electricaldegrees out of phase with its two immediate neighbors.

A group of six torches, T to T is then connected to these six voltagesources in a proper sequence. The proper sequence is that, regardless ofwhich terminal is connected to the first arc, the next terminal, movingeither clockwise or counterclockwise around the voltage source vectordiagram is connected to the next succeeding arc. Using this arrangementthe 4 center arcs, T to T,,, will sweep across the workpiece inpredictable response to the alternating magnetic fields created by theadjacent arcs.

The tendency for the two outer arcs, T and T to deflect outwardly fromthe group can be overcome through magnetic stabilization, or pointing ofthese arcs towards the four center arcs. Another preferred method ofcontrolling the outer arcs is to position U-shaped tubes or rods 48 inFIG. 4, in close proximity to the outer arcs and passing current throughthe tubes. To be most effective the current flow through the tubesshould be in a direction opposite that of the current in the arc.

The above-described system of a six-phase AC power supply withsequential connections to a six torch array allows for the effectiveheating of a wider area of the slab and does away with the need forelectrical connections to the workpiece. This system is more fullydescribed in application Ser. No. 565,340 entitled Method and Apparatusfor Sweeping Electric Arcs, filed on June 29, 1966, now U.S. Pat. No.3,336,460, issued Aug. 15, 1967. In addition, other suitable AC and DCarrangements of directionally stable arcs are set forth in thatapplication which are suitable for use in practicing the process of thisinvention. The six torch array shown may be easily added to inincrements of six torches to give lateral rows containing l2, 18, 24, 36torches. In this manner a slab of any width can be effectively arc alloyclad according to the process of this invention.

A mechanical side to side movement or oscillation of the box 28 can besuperimposed on the electromagnetically created sweeping of theindividual arcs to give a greater width and uniformity of heating.

When one side 24 of the slab 18 has been clad, the slab may be turnedover and reprocessed on the opposite side. The plate or sheet productobtained from a slab cladded on both sides will, of course, be itselfstainless clad on both sides. For some purposes, however, a single sidecladded product may be all that is needed for the particular useintended.

If the clad slab is to be converted to sheet product, the first step isusually to crop the ends of the slab since those parts may not have beenuniformly alloyed. Slight surface scale or slag resulting from the arcalloying process may then be removed, as by shot blasting. Theso-conditioned slabs may then be preheated and hot rolled to anintermediate thickness, say one-eighth of an inch. This stock can thenbe annealed, e.g., at about l,450 F. for 1 hour, although material hasbeen successfully self annealed in the coil. Descaling then followsusing, for example, sodium hydride for 20 minutes followed by nitricacid and hydrofluoric acid for 10 seconds. Shot blasting may also beemployed. After descaling, the surfaces can be inspected and spotconditioned.

The one-eighth inch stock can then be cold rolled directly to 0.060,0.040, or 0.025 inch thick sheet without intermediate anneals. In fact,direct cold reduction to foil gages is possible. The material coldrolled to these final gages much more like carbon steel than like astainless steel.

The stainless clad sheet was found to be substantially free of defects.Its mechanical properties more nearly resemble those of carbon steelthan stainless steel. Nominal tensile strengths were 22,000 psi yieldand 44,000 psi tensile with an elongation of 39 percent in 2 inches.Drawability and bend ductility were at least as good as type 430stainless steel.

The corrosion resistance of a 20 percent chromium clad product was atleast as good as type 430 stainless except at the edges of the sheet,which were not coated. The clad sheet was joined using conventionalresistance and arc welding methods.

When it is desired to achieve maximum surface quality and appearance ofthe resultant product, it is important that the entire lateral expanseof the slab surface to be treated be melted in one operation. Themelting and alloying of the entire lateral expanse at one time willallow for cooling and solidification of this lateral expanse at auniform rate. The result of such an operation is shown in FIG. 6. Therea slab 18 is shown after the creation of an arc alloyed clad layer 22according to this invention. Since the whole lateral expanse of the slabwas melted in one operation, the whole molten layer solidifieduniformly. Heat left the molten layer both directly downward byconduction into the cooler slab core and by radiation upwards throughthe slag-layer covering the surface. The result is a solidificationpattern marked by a parallel columnar grain structure 52. The uppermostsection of the layer cooled faster as a result of its proximity to thecooler environment above resulting in a smaller layer 54 of similarparallel columnar grains 59, but of a smaller size than the underlyingstructure. This parallel columnar grain structure has been found to becapable of rolling into sheet product free of banding and other surfaceimperfections.

When care is not taken to melt the entire lateral expanse of the slab,as when multiple passes of arc columns are used, the solidificationpattern will be closer to that shown in FIG. 7. There the structure isnot made up of parallel columnar grains across the entire surface, butrather only groups 56 and 58 of such parallel grains with anintersecting non-vertical structure 60 formed at the intersectionpasses. When rolled into sheet product, a banded" appearance will occurwith the bands located over the overlapping sections and runninglongitudinally down the sheet product. Such a banded product may beundesirable for applications where a high degree of uniform appearanceis required, although quite acceptable for many other applications.

While the invention has been described with reference to certainpreferred embodiments, it should be understood that certainmodifications can be made to the embodiments described without departingfrom the spirit and scope of the invention. For example, an alloyed slabcan be made by using multipasses or layers of alloying material and suchalloying materials or the base metals being treated may be varied.

What is claimed is:

l. A method for alloying a material into at least a selected portion ofthe surface of a body of a base material wherein a part of the basematerial becomes a part of the alloy and wherein an alloy layer ofsubstantially uniform depth and composition is formed on said selectedportion of the surface of said body, comprising melting a selectedcontinuous lateral expanse of the surface of said body onto whichalloying material is introduced by establishing a plurality of arcs atcurrent levels of from 300 to 10,000 amperes from non-consumableelectrodes provided in are devices which include a nozzle having anoutlet passage, confining the arc in the nozzle, in troducing gasthrough the nozzle and into the arc, the combination of said gas flowand said outlet passage nozzle diameter providing a directionally stablearc effluent, positioning said directionally stable arc effluentsproximately to each other so that the magnetic field created around eachof said arcs effects the arcs in immediate proximity therewith,energizing said arcs such that an AC voltage is applied at least toalternating arcs to produce said interacting magnetic fields among saidmulti arcs, spacing said nozzles from about /4 to about 1 nozzle outsidediameter from the surfaces of said body such that the length of saidarcs may be swept across said selected surface portion and, varying atleast one of said are current, said gas flow, and said arc length tocontrol the sweep of said arcs, and moving said body and said pluralityof electric arcs relative to one another in a direction from one end ofthe body to another to advance the molten zone of alloyed material tosuccessive lateral expanses on said body, while allowing already moltenlateral expanses to cool under conditions substantially uniform acrossthe entire selected lateral expanse.

2. The method of claim 1 in which all of said arcs are ac. arcs.

3. The method of claim 1 in which a chromium containing material ismelted and alloyed into a steel slab to produce a stainless steel cladla er on said slab.

4. The method 0 claim 1 in which granulated chromiumcontaining materialis disposed over a surface of said slab and in which said so-coated slabis preheated to a temperature in the range l,400 to l,600 F prior to themelting operation.

5. The method of claim 4 in which granulated ferrochromium containingabout 70 percent by weight chromium is disposed over the surface of acarbon steel slab and the socoated slab melted to produce a clad layerhaving a chromium content of from 14 to 20 percent chromium.

6. The method of claim 4 in which a sodium silicate binder is used tohold the granulated material to the slab.

7. The method of claim 1 in which the forward rate of progress is atleast three inches per minute.

8. The method of claim 1 in which at least one material melted andalloyed into the steel slab is selected from the class consisting ofchromium, nickel, iron, columbium, manganese, molybdenum, tantalum andtitanium.

1. A method for alloying a material into at least a selected portion ofthe surface of a body of a base material wherein a part of the basematerial becomes a part of the alloy and wherein an alloy layer ofsubstantially uniform depth and composition is formed on said selectedportion of the surface of said body, comprising melting a selectedcontinuous lateral expanse of the surface of said body onto whichalloying material is introduced by establishing a plurality of arcs atcurrent levels of from 300 to 10,000 amperes from non-consumableelectrodes provided in arc devices which include a nozzle having anoutlet passage, confining the arc in the nozzle, introducing gas throughthe nozzle and into the arc, the combination of said gas flow and saidoutlet passage nozzle diameter providing a directionally stable arceffluent, positioning said directionally stable arc effluentsproximately to each other so that the magnetic field created around eachof said arcs effects the arcs in immediate proximity therewith,energizing said arcs such that an AC voltage is applied at least toalternating arcs to produce said interacting magnetic fields among saidmulti arcs, spacing said nozzles from about 1/4 to about 1 nozzleoutside diameter from the surfaces of said body such that the length ofsaid arcs may be swept across said selected surface portion and, varyingat least one of said arc current, said gas flow, and said arc length tocontrol the sweep of said arcs, and moving said body and said pluralityof electric arcs relative to one another in a direction from one end ofthe body to another to advance the molten zone of alloyed material tosuccessive lateral expanses on said body, while allowing already moltenlateral expanses to cool under conditions substantially uniform acrossthe entire selected lateral expanse.
 2. The method of claim 1 in whichall of said arcs are a.c. arcs.
 3. The method of claim 1 in which achromium containing material is melted and alloyed into a steel slab toproduce a stainless steel clad layer on said slab.
 4. The method ofclaim 1 in which granulated chromium-containing material is disposedover a surface of said slab and in which said so-coated slab ispreheated to a temperature in the range 1, 400* to 1,600* F prior to themelting operation.
 5. The method of claim 4 in which granulatedferrochromium contaIning about 70 percent by weight chromium is disposedover the surface of a carbon steel slab and the so-coated slab melted toproduce a clad layer having a chromium content of from 14 to 20 percentchromium.
 6. The method of claim 4 in which a sodium silicate binder isused to hold the granulated material to the slab.
 7. The method of claim1 in which the forward rate of progress is at least three inches perminute.
 8. The method of claim 1 in which at least one material meltedand alloyed into the steel slab is selected from the class consisting ofchromium, nickel, iron, columbium, manganese, molybdenum, tantalum andtitanium.